Plastic foamed molded articles are widely used throughout the industry due to properties such as excellent lightweightness, cushioning, heat insulation, moldability, and energy saving performance thereof. Polymers such as polystyrene, polyolefin, or polyvinyl chloride are amorphous and have a high melt viscosity, and their viscosity is changed little by temperature change. Such polymers are easily foamed, and thus are widely used as heat insulating materials, structural materials, cushioning materials and packaging containers, etc. However, the above-described polymers are vulnerable to fire, release environmental hormones, and have poor physical properties.
On the other hand, polyester is an eco-friendly material having excellent mechanical properties, heat resistance and chemical resistance, and thus is applicable to various fields requiring light weight and excellent physical properties.
However, polyester is difficult to mold by melting, extruding, and foaming due to being a crystalline resin. In this respect, with the development of technology, it has become possible to manufacture foamed molded articles through a foaming process using polyester. For example, U.S. Pat. No. 5,099,991 discloses a technology of producing a foamed molded article by adding a cross-linking agent to a polyester and extrusion-foaming a mixture thereof.
However, since a polyester resin has a high melt viscosity, it is important to control a viscosity of a molten resin when continuous extrusion foaming is carried out, but when additives are mixed, it becomes more difficult to control the viscosity of the molten resin. Specifically, in the case in which a separate functional additive is mixed in an extrusion-foaming process of the polyester resin, when the viscosity of the molten resin is low, bubbles generated by a foaming agent escape to the outside rather than being collected in the resin, and when the viscosity of the molten resin is high, it is difficult for the generated bubbles to be collected in the resin, causing a low foaming ratio. Thus, the foaming ratio may be lowered, and the appearance of the foamed molded article may become poor.    Prior Art Document: U.S. Pat. No. 5,000,991